LONDON, Sept 21 (Reuters) - Greenpeace said on Thursday anti-pollution activists had boarded a ship in the Thames Estuary containing Volkswagen diesel cars being imported into Britain in an attempt to force the vessel to return to Germany.

More than 40 volunteers are also trying to immobilize cars parked at Sheerness port in the southern English county of Kent by removing the keys and are sticking labels to engines, calling on VW to stop making diesel vehicles, the environmental pressure group said.

VW admitted to cheating diesel emissions tests in 2015, forcing the firm to pay compensation to U.S. motorists and leading to a regulatory crackdown on diesel, including new levies and a ban on sales in Britain and France from 2040.

No immediate comment was available from VW. (Reporting by Costas Pitas; editing by Stephen Addison)